(1) Field of the Invention
Polyhydroxyalkanoates (PHAs), particularly polyhydroxybutyrate (PHB), are biodegradable polyesters derived from renewable resources and have shown excellent promise as environmentally friendly substitute for polypropylene (PP). This invention aims to reduce the inherent brittleness of PHA's (PHB), while retaining their attractive stiffness and strength, by incorporating functionalized (reactive) rubbers. This provides significant improvement in toughness with minimum compromise in the stiffness. Preferably clays are provided in the composites.
(2) Description of Related Art
Many semi-crystalline polymers like PHB, Nylon and PP exhibit very attractive strength and ductility at room temperature and under moderate rates of deformation. However, they become brittle under severe conditions of deformation such as low temperature or high strain rates, and can undergo a sharp ductile-to-brittle transition (Lu, et al., Journal of Applied Polymer Science, Vol. 76, 311-319 (2000)). In the brittle regime a crack can propagate with little resistance. Because of this poor performance at extreme conditions there has been considerable commercial and scientific interest in the toughening of semi-crystalline thermoplastics. An extensive literature is now available on the toughening of commodity as well as engineering polymers such as polyethylene (Bartzcak et al., Polymer, 40, 2331-2346 (1999); Bartzcak et al., Polymer, 40, 2347-2365 (1999); and Macromol. Mater. Eng. 289 360-367 (2004)), polyamide (D. M. Laura et al., Polymer 42, 6161-6172 (2001)), polypropylene (Ismail, H. and Suryadiansyah, Journal of Reinforced Plast. And Composites, 23, 6, 639-650 (2004); Van der Wal et al., Polymer, 39, 26, 6781-6787 (1998); and Van der Wal et al., Polymer, 40, 6031-6075 (1999)) and polyvinylchloride [Ishiaku et al., Journal of Applied Polymer Science, Vo. 73, 75-83 (1999); and Ishiaku et al., Journal of Applied Polymer Science, Vol. 69, 1357-1366 (1998)).
Under proper conditions and using appropriate compatibilizers, synergistic effects arise to create high impact toughened polyolefins (TPO). Typically, a stiff filler material is incorporated into this TPO matrix to overcome the lost stiffness and strength. These fillers were conventionally glass fibers (Mehta et al., Journal of Applied Polymer Science, Vol. 92, 928-936 (2004)) but recent developments and results (Okada, O., et al., Mater Res Soc. Symp Proc., 171, 45 (1990); Pinnavaia, T. J., et al., ACS Symp Ser 622, 250 (1996); Messersmith, P. B., et al., Chem Mater 6, 1719 (1994); Yano, K., et al., J. Poly Sci Part A: Polym Chem., 31, 2493 (1993); Vaia, R. A., et al., Chem Mater 5, 1694 (1993); Wang, Z., et al., Chem Mater 10, 3769 (1998); Ke, Y., et al., J. Appl Polym Sci 71, 1139 (1999); Hasegawa, N., et al., J. Appl. Polym. Sci. 63, 137 (1997); and Mohanty, A. K., et al., Proceedings of 9th Annual Global Plastics Environmental Conference (GPEC 2002), Feb. 26 & 27 (2003), Detroit Mich., Society of Plastics Engineers, Plastics Impact on the environment, Full paper published in the Proceedings 69-78, (2003)). Use of a nanoclay has been described in TPO's.
The incorporation of rubber particles into a brittle thermoplastic matrix is known to improve the impact properties and the toughness of the polymer (Amos, J. L., et al., U.S. Pat. No. 2,694,692 (1954); Baer, et al., U.S. Pat. No. 4,306,040 (1981); and Patel, P., et al., Rubber-toughened thermoplastics, Brit. Pat. (1978)). Under proper conditions and using appropriate compatibilizers, synergistic effects arise to create high impact toughened blends. But, adding low modulus rubber particles to the polymer lowers the stiffness and strength and this reduction in rigidity significantly lowers the scratch/mar resistance of the resulting blends. This problem has hindered the growth of rubber-toughened thermoplastics in the automotive industry. Hence, to overcome this brittleness, high modulus fillers like clay are incorporated into the toughened blend which, with optimal processing and chemistry, can regain this lost strength and stiffness (Suzuki, K., et al., Thermoplastic resin nanocomposites with good heat and impact resistance and rigidity for automobiles, Jpn. Kokai Tokyo Koho (2004); Ito, T., et al., Manufacture of polyolefin compositions for automobile parts with improved rigidity and heat resistance, Jpn. Kokai Tokkyo Koho (2004): and Maruyama, T., et al., Rubber nanocomposites containing layered clay minerals well dispersed therein, Jpn. Kokai Tokkyo Koho (2004)). General Motors and supplier partners recently launched a nanocomposite TPO-based step-assist which was the first instance of a nanocomposite material being used in automotive exterior applications (http://www.scprod.com/gm.html).
However PP and subsequently TPO are both non-biodegradable and also petroleum-based. Vast amounts and varieties of such plastics, notably polyolefins, are currently produced from fossil fuels, consumed and discarded into the environment, ending up as un-degradable wastes. Manufacturers are looking for alternative eco-friendly green materials that can replace these non-renewable-resource based non-biodegradable materials. Numerous recent federal acts and executive orders encourage the development of biobased products to assist in ‘greening’ the country through recycling and waste-prevention. These green biomaterials not only protect the environment and reduce greenhouse gasses but also increase national security by reducing dependency on foreign oil for our needs.
Another route to overcome the inherent brittleness of polyhydroxybutyrate is by using polyhydroxybutyrate-hydroxyvalerate (PHBV) copolymers, which have low levels of valerate. However, PHBV exhibits lower melting point than PHB and so narrows the utilization temperature range of the composition. PHBV is also costlier than PHB and this hinders its scope and usage.
U.S. Pat. No. 5,714,573 to Randall et al describes polylactide polymer compositions. The present invention does not use lactide polymers.
Based on the above literature, the following problems were identified with conventional toughened polymers:                1) The incorporation of rubber particles into a brittle thermoplastic matrix is known to improve the impact properties and the toughness of the PHA polymer but only under proper conditions and using compatiblizers.        2) Adding elastomer particles to the PHA polymer lowers stiffness and strength and this reduction in rigidity significantly lowers the scratch/mar resistance of the resulting blends.        3) Stiffness and strength of the PHA polymer can be regained by adding a stiff reinforcement like nanoclay but property improvements are only achieved if optimum dispersion and compatibility are created.        4) Clay is inherently hydrophilic and hence does not mix with the PHA polymer matrix. This leads to agglomeration and poor properties and this has to be overcome by modifying the clay surface.        5) Conventional TPO's are based on non-renewable resources and hence are not sustainable or ecofriendly and there is a need for alternative eco-friendly green materials that can replace these non-renewable-resource based non-biodegradable materials.        6) Performance limitations and high cost however have limited these PHA biopolymers to niche markets.        7) PHB is typically a bacterial biobased polymer. It has mechanical properties very similar to the matrix polymer PP in TPO. However, PHB's main drawbacks are its brittleness and thermal instability.        